A 38 kHz upward-facing echosounder was deployed on the seafloor at a depth of 875 m in Monterey Bay, CA, USA (36 • 42.748' N, 122 • 11.214' W) This 18-month record of acoustic backscatter was compared to oceanographic time series from a nearby data buoy to investigate the responses of animals in sound-scattering layers to oceanic variability at seasonal and sub-seasonal time scales. Pelagic animals, as measured by acoustic backscatter, moved higher in the water column and decreased in abundance during spring upwelling, attributed to avoidance of a shoaling oxycline and advection offshore. Seasonal changes were most evident in a non-migrating scattering layer near 500 m depth that disappeared in spring and reappeared in summer, building to a seasonal maximum in fall. At sub-seasonal time scales, similar responses were observed after individual upwelling events, though they were much weaker than the seasonal relationship. Correlations of acoustic backscatter with oceanographic vari- * Corresponding author ability also differed with depth. Backscatter in the upper water column decreased immediately following upwelling, then increased approximately 20 days later. Similar correlations existed deeper in the water column, but at increasing lags, suggesting that near-surface productivity propagated down the water column at 10-15 m d −1 , consistent with sinking speeds of marine snow measured in Monterey Bay. Sub-seasonal variability in backscatter was best correlated with sea-surface height, suggesting that passive physical transport was most important at these time scales.Physical variability is a fundamental feature of the ocean's pelagic habi-2 tat, and the responses of pelagic organisms to this variability play a large role 3 in determining their distribution, abundance, and survival. As such, under-4 standing the effects of physical change and variability on ocean life has long 5 been recognized as a central challenge in oceanography and marine ecology.6Physical variability can influence organisms at the individual or population 7 level, and its effects can be direct (e.g. advection) or indirect (e.g production 8 fertilized by upwelled nutrients). 9 "Physical-biological coupling" has most often been studied in the plank-10 ton, whose abundance and distribution are closely tied to physical variability 11 (Platt and Denman, 1975). However, because ocean currents are variable 12 across a wide range of spatial and temporal scales, the division between 13 14 physical processes is thus expected to extend from zooplankton to micronek-15 ton: animals roughly 2-10 cm in length with swimming abilities in between 16 those of drifting plankton and freely swimming nekton (Brodeur and Yama-17 mura, 2005). These include krill, pelagic shrimps, small squids, and fishes 18 such as myctophids. 19 Assemblages of micronekton are important constituents of deep scatter-20 ing layers (DSLs, Dietz 1948; Barham 1956). DSLs are layers of elevated 21 animal biomass, and consequently acoustic backscattering, which are found 22 worldwide in ...